In order to build a high-resolution infrared thermopile sensor array, the number of individual thermopile elements, i.e., the number of pixels, must increase and the geometric dimensions of the pixels themselves must decrease. The thermopile sensor arrays originally available on the market had only a few pixels on (e.g. 8×8 pixels or 16×16 pixels), wherein the individual pixels were rather large (e.g. 150 . . . 300 μm×150 . . . 300 μm). There was therefore sufficient space on the sensor chip (e.g. made of silicon) to accommodate a small number of amplifiers or low-pass filters on the chip next to the thermopile sensor array.
Currently used thermopile sensor arrays with larger pixel counts require a reduction in the dimensions of the pixels to a side length of 100 μm, or even down as far as 25 μm. However, the pixels that are becoming ever smaller due to the increasing integration density have the disadvantage that they generate a smaller signal proportional to their surface area. This means that if a pixel is halved in size, only a quarter of the signal strength is available for further processing.
The resulting ever smaller signal voltages, which are usually in the nV-range up to a few μV, always require greater gain factors, so that signals can be further processed without additional noise or other interference outside of the actual sensor housing. However, the consequence is an ever-smaller signal-to-noise ratio.
The necessary signal amplification due to the low signal voltages is at least on the order of several 1000, usually even over 10,000, in order to raise the signal voltage far enough that it can be passed to other modules and further processed. The analog amplifiers with high gain classically used for signal amplification require multi-stage amplifiers with relatively large space requirements and in addition, these amplifiers have a significant power consumption. This means that the power losses and thus the self-heating increase, which in turn leads to an overall increase in the measurement error of the thermopile sensor array.
In the meantime, infrared thermopile sensor arrays, which are manufactured on a chip using silicon micro-mechanics, have come to be known in different designs. In these sensor arrays, part of the signal processing takes place on the chip, but there are only a few preamplifiers and a common multiplexer present, which outputs the signals from all the pixels. The thermopiles have so-called “hot” contacts on an increasingly smaller infrared reception surface and so-called “cold” contacts on a heat sink at the edge of the respective pixel. The signal voltage generated by the thermopile is directly dependent on the temperature difference that can be achieved between the “hot” and “cold” contacts.
For example, in a thermopile sensor array according to WO 2006/122529 A1 one preamplifier and one low-pass filter are integrated on the chip per row of the sensor array. For high-resolution sensor arrays with multiple rows and columns, however, this is not sufficient. For example, in the case of an array of 64×64 pixels only 64 preamplifiers and 64 low-pass filters are used. The achievable noise bandwidth would be up to 64 times higher than necessary. However, since the noise increases with the square root of the noise bandwidth, the noise could be reduced by up to 8-fold, or the thermal resolution could be improved by up to 8-fold.
However, no measures are specified for a power-saving and space-saving solution for signal amplification per signal channel.
Still, in JP 2004-170375 a thermopile sensor array is disclosed, which has only a single preamplifier.
In DE 103 22 860 B4 a circuit arrangement is described for reading out electronic signals from high-resolution thermal sensors with preamplifers, which are arranged in front of a multiplexer. To reduce the significant power loss of the individual parallel operating preamplifiers, these are cyclically switched off to conserve power.
By precisely this measure, however, the desired high temperature resolution is not achieved, because the preamplifiers before the multiplexer can only deliver the expected result if, at the same time, the noise bandwidth of the sampled signal is reduced in proportion to the number of parallel amplifier channels. This is not possible, however, if the signal amplified via the preamplifiers is not continuously “integrated” by means of a low-pass filter for limiting the noise bandwidth, or smoothed, for example, using a low-pass element.
Document EP 2 587 234 A1 discloses a thermopile infrared sensor with a signal processing circuit, in which the signals of the individual pixels are forwarded without band limitation, or intermediate preamplifiers.
In all the above solutions, thermopile infrared sensor arrays are described, without, however, measures being proposed for signal processing at a higher integration density on the chip. In particular, there are no proposals for reducing the noise bandwidth while, at the same time, retaining minimum space requirements and minimum power loss.
The known solutions have an inadequate thermal resolution, because only a single one or very few preamplifer channels have been integrated on the sensor chip, resulting in a high signal-to-noise bandwidth and at the same time, a poor signal/noise ratio.
A high integration density requires the pixel size and the so-called pixel pitch, i.e. the center-to-center distance between the thermopile pixels, to be reduced in order to accommodate more pixels on the same chip surface area. In addition, as well as a high geometric resolution a high thermal resolution is also desirable, i.e. a large signal/noise ratio and a low noise-limited temperature resolution NETD (Noise Equivalent Temperature Difference).
Because of the smaller reception surface area and because of the resulting small distance between the “hot” and “cold” contacts of the thermopile pixels, a reduction in the pixel size also gives rise to a reduction of the sensor signal emitted from each thermopile pixel, resulting in a lower signal/noise ratio, poorer thermal resolution and a reduced measurement accuracy.
In principle, it is possible to reduce the size of thermopile cells (thermopile pixels) and integrate ever larger numbers of pixels on the sensor chip. For example, 16×16, 32×32, 64×64, 128×128 thermopile pixels or more are implemented on a sensor chip. The signal voltages of the individual thermopile pixels would need to be multiplexed using m×n-addressing and MUX switches, i.e. to be routed onto a common serial signal line to one amplifier per array, or via a common serial interface per row or column.
Since the minimum signals still to be resolved from thermopile pixels with further reduced pixel dimensions for many applications will now be in the nV range, the signals must be amplified to a sufficiently high level and further processed on the chip itself, so that they cannot be affected by electrical interference sources both outside of and within the sensor chip.
Typical amplification factors of 10,000 or more are required in the known solutions, in order to amplify the sensor signals of such small thermopile pixels up to several mV, so that the sensor signals after the MUX (multiplexer) can be output from the sensor chip on a fast serial analog output—or converted into digital signals by a fast AD-converter integrated on or directly adjacent to the sensor chip.
The disadvantage of this is that the bandwidth of these preamplifiers located after the multiplexer must be very high in order to still transmit the sensor signals of many consecutively sampled thermopile pixels with frame rates from several Hz up to several 10 Hz.
To achieve this requires at least m×n times the frame rate in the case of one preamplifier per array, or m times the frame rate for an array with m columns and one amplifier per column. However, the noise of the system also increases and the temperature resolution NETD deteriorates at the same time proportional to the square root of the (noise) bandwidth.
Stable and high-precision amplifiers with high gain require a plurality of amplifier stages, need a relatively large amount of space on the sensor chip and also have a significant power consumption with correspondingly high waste heat, which in turn further reduces the achievable signal voltage of the thermopile elements.
For this reason, on the severely limited space of a sensor chip it is not possible to accommodate many such large amplifiers in addition to the pixels.
Finally, Kassovski ET AL: “Miniaturized 4×16 Thermopile Sensor with Integrated on Signal Conditioning on Chip”, Proceedings of IRS 2011, page 57, XP055300518, describes a row array with 4 rows each of 16 elements, hence a total of 64 pixels per line. In this thermopile sensor, 64 signal processing channels are provided for 64 pixels, thus one signal processing channel per pixel, wherein each pixel contains 80 contact temperature sensors.
Each signal processing channel comprises a low-noise amplifier with unknown gain, a 16-bit 2nd order Delta Sigma A/D converter and a digital low-pass filter, wherein the signal processing is carried out on the same chip. The buffering of the measurements is carried out in a RAM on the same chip.
Since this small 64-element array has only 4 rows and a row spacing of 220 μm, two signal processing channels can be readily accommodated on both sides of each of the pixels. Even in a two-dimensional array with 16 rows, 8 signal processing channels would then have to be accommodated on each side, with 64 rows therefore, 32 instead of two channels would be needed per side. For two-dimensional arrays with very many elements this would require a chip with a very large surface area, with associated high costs. Also, no information is given as to how, for arrays with a much greater pixel count and very small pixel pitch of <200 μm or preferably <100 nm, the signals from so many pixels could be processed in parallel and routed to the signal output.
At an operating voltage of 3V the array also consumes a current of 4 mA, which means that a power loss of 12 mW is generated, which in the case of small thermopile infrared sensor arrays would still be acceptable. In the case of larger sensor arrays with larger pixel counts, however, this would look very different, because when reading out such an array with the same current consumption of 4 mA, the resulting electrical power loss would then be much too large.
The equivalent pixel-proportional power consumption produced by a 64×64 sensor array with 64 times more signal processing channels would be almost 250 mA, i.e. a power loss of 0.75 W, and for a 128×128 sensor array a power consumption of almost 1 A at a power loss of 3 W.
Such high power losses, however, exhibit the following disadvantages in thermopile infrared sensor arrays:
if such a high power loss were to be integrated monolithically on the same sensor chip, this would lead to the intrinsic heating of the sensor chip and, in particular after being powered on, to a kind of thermal shock of the sensitive thermopile elements. This results in a lower measurement accuracy of the thermopile elements and a short battery life in portable devices.
In WO 2017/059 970 A1 a high-resolution thermopile infrared sensor array with monolithic integrated signal processing is disclosed, in which the signals of each signal processing channel generated by the sensor array via preamplifers and a downstream analog-to-digital converter are buffered in a memory. The selection of the respective signal processing channel is performed by means of a signal multiplexer.
Document U.S. Pat. No. 9,270,895 B2 discloses a method and a device for highly dynamic image generation, in particular for generating a digital representation of a scene with a two-dimensional matrix of IR sensors. For this purpose, each pixel is assigned an analog-to-digital converter and an m-bit counter.
Document US 2006/0 243 885 A1 relates to an image sensor and a method for controlling the same, wherein the object is to create an improved image sensor with small dimensions, in which the light collection array and the A/D converters are arranged on one chip. It is designed to implement a high-speed controller. This is achieved by dividing the image sensor array into a plurality of sub-arrays, which are each assigned to an A/D converter with an associated sub-array controller.
Document U.S. Pat. No. 8,179,296 B2 relates to a method and a device for the digital readout of a sensor array, which is connected to the input of an A/D converter array. The analog/digital array can be used for IR-image sensor applications with high areal resolution (small pixels) and high signal-to-noise ratio.
In JP 2004-170 375 A, a thermopile array sensor is described, in which the white noise, caused by ambient temperature changes and the 1/f noise of the DC amplifier, is to be eliminated. This is achieved by means of a shielded compensation thermopile per row of the array. For signal processing an op-amp is used, which processes the difference between the measurement and compensation signal.
Document WO 2006/122 529 A2 relates to a thermopile infrared sensor array, in which the membrane under each thermopile sensor element is exposed by etching and in which preamplifiers with low-pass filters are provided for at least every fourth, preferably for every column or row of sensor elements.
Document EP 2 587 234 A1 relates to an IR sensor for suppressing changes in the to signal-to-noise ratio as a result of the warming of the chip, by heating the cold contact. On the other hand, thermal radiation or thermal conduction through the gas medium also leads to a heating of the hot contact. Each IR detector in the array is associated with a MOS transistor as a pixel selection switch, and with a plurality of vertical reading lines and horizontal signal lines. Through a combination of series and parallel connection of thermopiles the signal-to-noise ratio can be improved, wherein the output signal from each thermopixel is utilized.
Finally, DE 103 22 860 B4 relates to a circuit arrangement for reading out electronic signals from high-resolution thermal sensors, in which the signals from a plurality of sensor elements are each serially read out via one or a small number of data lines using a multiplexer, and an amplifier is connected between each individual thermal sensor element and the multiplexer. To reduce the thermal load the amplifiers can be switched on and off cyclically.